Cooling electrical apparatus



July 18, 1944. E, VENABLE COOLING ELECTRICAL APXARATUS Filed Sept. 27, 1940 /7I L\\l' 1\\ 1 llllllll/l//lllllllA n INVENToR f' erso Ve/m/e.

WITNESSES:

Patented July 18, 1944 COOLING ELECTRICAL APPARATUS Emerson Venable, Wilkinsburg, Pa., assigner to Westinghouse Electric Manufacturing Company, East Pittsburgh, Pa., a corporation of .Pennsylvania Application September 27, 1940, Serial No. 358,650

3 Claims.

This invention relates to electrical apparatus, and more particularly, to the cooling of electrical apparatus.

Electrical apparatus when in operation develops a quantity of heat depending on its electrical resistance, construction and the amount of current flowing. The heat must be dissipated in some way, generally to the atmosphere. The problem of heat dissipation is particularly vital in apparatus which has substantially no moving parts, such for example, as transformers and reactors.

Heretofore, cooling of stationary or non-moving electrical apparatus, such as the transformers above-mentioned, was accomplished by the use of some fluid such as a liquid or gaseous heat transfer medium. In transformers, the greater proportion of installations employ petroleum oils as the heat dissipating medium. Halogenated hydrocarbons have also been put into transformers in some instances. However, due to the fact that petroleum and related hydrocarbons have a heat conductivity of the order of 0.00035 calorie per second per square centimeter per centimeter thickness per degree centigrade, which is about 1smooth of the heat conductivity of a metal such as copper, little or no reliance can be placed upon the dissipation of the transformer heat by thermal conduction. The greater proportion of heat is dissipated from an oillled transformer by convection currents of oil which flow past or through electrical elements to pick up the generated heat, and this heat is carried to the relatively cooler exterior walls of the transformer casing by the convection currents where it is given up. Accordingly, the construction of transformers employing liquid cooling of this type has required considerable emphasis on the design of the coils and the core to provide passages to facilitate maximum convection flow of the fluid therethrough. This provision of cooling apertures increases the space factor and does not result in the most economical arrangement of the material for a given capacity or for the highest electrical efilciency.

Petroleum oil is exceedingly inflammable, and its use in transformers in densely populated areas and certainpotentially dangerous localities is greatly restricted or not permitted. Therefore, transformers operating with air or gas cooling are employed in such critical places. An even greater emphasis must be placed on the provision of appropriate cooling passages for air than is required for oil. The same disadvantages in apparatus construction and electrical efliciency appear with air or gas cooling just as they do with the liquid cooling construction.

In addition, both oil and air cooled transformers may be subject to unsatisfactory performance or failure due to deteriorating phe nomena. Oil is subject to break down under arcing conditions with the formation of carbonaceous deposits, sludges and inflammable or explosive gases. results in a chemical breakdown of the oil and great care is required to prevent this occurring.

However, oil does have good electrical insulating properties and is preferable on this account to air as a cooling medium.

The employment of air as a cooling medium for electrical apparatus requires precautions not necessary with oil cooling, in order to avoid the deteriorating effect of moisture and highly reactive gases present therein. 'I'he electrical elements within -the apparatus must carry a greater amount of solid insulation, since the insulation must withstand the total electrical stresses without much dielectric assistance from the air. Due to thicker insulation, the air-cooled transformer elements must operate at a much higher thermal gradient to dissipate heat as compared to a liquid or oil-cooled transformer. This higher temperature results in impaired electrical efllciency for the apparatus.

According to this invention, it is proposed to embed the electrical elements of the apparatus being cooled in a solid inorganic substance which provides for a good thermal dissipation and a high degree of electrical insulation.

It is a purpose of the invention to cast molten mineral or inorganic dielectric material directly around the electrical elements which are subject to heating in order to provide the best possible transfer of heat from the elements to the cooling mineral. The heat transfer dielectric mineral extends in a homogeneous mass to the wall of the casing enclosing the electrical elements and contacts the casing wall closely. Therefore, a very good overall thermal 'conductivity will be had between the electrical element being cooled and the exterior casing.

In apparatus constructed according to the in` vention, reliance is'placed almost entirely upon thermal conductivity to dissipate the heat developed and none on convection currents. Ac-

cordingly, the construction of the apparatus will not be influenced by the necessity to obtain suitable passages for conventional flow of fluid through the electrical elements, but instead the The contact of oxygen with oil primary emphasis may be placed on an economical construction of apparatus ior a given capacity, and to achieve good electrical efilciency in operation.

It has been found that certain inorganic salts or minerals have a thermal conductivity which is from 50 t0 80 times greater than that of petroleum. These minerals likewise have an exceedingly high electrical insulating value. Ordinarily, materials which have high thermal conductivity likewise exhibit good electrical conductivity. However, a few crystalline minerals or salts are exceptions to this general rule. Among these minerals are fluorite (CaFz) halite (NaCl) sylvite (KCl), villiaumite (NaF), and potassium fluoride (BT). These minerals have thermal conductivities of 0.026, 0.0166, 0.0166, 0.025 and 0.016, respectively, in calories per second per square centimeter per centimeter thickness per degree centigrade.

The electrical resistivity of these ve specific minerals is exceedingly high at all temperatures up to a point just below their fusion temperature, thus at 400 C. the resistivity o! halite is 8.27)( 10'I ohms, while at 520 C. the resistivity of sylvite is 467x 10 ohms. At room temperature the resistivity is of the order of 101" ohms for all these minerals. It will be seen that these minerals will provide excellent electrical insulation at temperatures much above the ordinary operating temperatures of electrical apparatus ,of the type contemplated.

This invention is predicated upon casting the above type of minerals or salts in a fused state around the electrical elements to provide for good heat transfer. While the fusion temperature of fiuorite alone is considerably above that of the melting point of the good conducting metals, such as copper, it is possible to employ a mixture oi iluorite with other salts, the mixture having a much lower melting point, for this purpose. Both halite and sylvite may be employed as pure salts, since their fusion temperature is below that of the melting point o! electrical members or elements ordinarily employed. However, the eutectic of halite and sylvite, or other mixtures thereof, having lower melting points as compared to the melting temperature of the single salts, will be preferable for many reasons. The fusion temis 790 C. A mixture of 50% halite and 50% sylvite has a melting point of 660 C. Owing to the relatively greater cost of sylvite as compared to halite, a 70% halite-30% sylvite mixture having a melting temperature of 697 C. may be a more economical mixture, without much increase in temperature required to produce fusion. Various mixtures of any two or all of these salts may be prepared in order to obtain a wide range of selected melting points. The electrical and thermal conductivities of the salts will remain satisfactory no matter in what proportions the mixtures of the minerals are combined.

In addition, the cast mineral body will effect a permanent and rigid spacing of the several electrical members both from each other and the enclosing casing. Vibration during operation of ie apparatus will be damped and greatly reduced. Short-circuiting or grounding will be obviated. Thus, a much more satisfactory electrical construction will be available to the electrical industry.

A solidified inorganic filling having a thermal conductivity of 0.01 calorie in c. g. s. units or higher and with dielectric properties Similar to ture of halite is 802 C., while that of sylvite the five inorganic salts or minerals above indicated, will be satisfactory for the cooling of electrical apparatus. Such minerals will withstand considerable overloads for prolonged periods of time without failure. in the case of concentrated heating due to arcing or short-circuits, the minerals may fuse and upon termination of the arcing, resolidify into the original condition.

It will be recognized that the inorganic filling should have a fusion temperature below that at which the electrical elements deteriorate or fail. A mineral fusing above the melting point oi copper, for example, would be unsatisfactory for casting purposes in connection with conductor elements of this metal. For this reason, a'high melting temperature mineral, such as quartz, would not be desirable in most instances.

An additional requirement of the inorganic filling material is that the fused filling must recrystallze on solidifying. An amorphous fusion product generally exhibits greatly decreased heat conductiv'ty. For example, quartz crystals have some ten times as great thermal conductivity as compared to amorphous fused quartz. A somewhat similar hcat conductivity ratio exists with other minerals in the crystalline the amorphous state. Ii the solidified mineral recrystallizes on cooling :from` the melt, it will retain the high conductivity of the natural crystal. The five minerals above listed recrysta'ilize after fusion and retain good thermal condire The invention will u I' il moet readily from the following detailed descriptiz'in thereof when considered in connection with the accompanying drawing, which isvrepresentative of one embodiment of the invention, in which the single figure is an elevational view partly in section. of the invention applied to a transformer.

Referring to the figure of the drawing, I0 is a transformer consisting of a bottom plate I2, side plate il and top plate I6 welded into a substantially hermetically sealed unit by welding as at I8. An aperture in the top plate I6 sealed by a closure member 20 is provided for the introduction of the molten solid dielectric cooling material. It is particularly desired to provide 'a hermetically sealed construction in order to prevent the entry of moisture and other deteriorating materials into the interior thereof.

Attached to and passing through the top plate I5 are two terminal conducting bushings 22 and 24 through which electrical current may pass to conductors 26 to the operative electrical elements. The operative electrical elements or i members consist of a laminated core 30 of magnetic material and electrical coils 32 placed thereabout with the view to obtaining the best construction and electrical efficiency. The coils 32 are mounted upon insulating sleeves 36 about the legs of the core 30. The coils 32 carry relatively thin insulation 34. There is no particular need to have any heavy insulation about the coils 32, but simply a thin layer in order that the thermal conductivity be a maximum.

In View of the fact that the cooling mineral to be cast about the electrical apparatus will withstand exceedingly high operating temperatures as disclosed hereinbeiore. it would be advisable and highly desirable to employ an inorganic or high temperature insulation about the coils 32 and for sleeves 35. Such materials as mica, asbestos, lead berate, glass and other high temperature insulation exemples of suitable coil and sleeve insulation.

A supporting structure for the electrical members and some means ensuring a slight coil separation should be provided in order that the fused mineral may properly fiow in and around the electrical elements. Spacing members of mica, porcelain or other high temperature material will be satisfactory for` this purpose. When the electrical members have been installed within the casing upon such supports and the top I8 welded in place, the electrical elements will be in a condition to receive the molten mineral charge which is intended to provide for cooling and spacing.

With the electrical elements properly supported and spaced for the reception of the molten charge, the contents of a crucible or other receptacle containing the molten salt or eutectic or mixture may be poured in through the opening at 20. It may be desirable to preheat the entire transformer assembly to a temperature approaching that of the molten salt in order to provide for good and homogeneous filling of the casing. After the molten salt has been introduced, cooling from the bottom plate l2 will provide for a regular crystalline solidification of the molten salt beginning at that point. A slow cooling will provide for a dense and uniform crystalline mass 40 of salt filling all the interstices between the coils, and closely adhering to the faces of the electrical elements and to the core 30. Furthermore, the salt will adhere to or closely contact the side H and bottom I2 of the transformer which will facilitate the transfer of heat in the completed apparatus. A slight shrinkage cavity 42 will be evident after solidification of the salt.

If it be desired to completely fill. the transformer from top to bottom, a shrink head may be applied above theside u of the casing before the top plate I6 has been attached thereto and the molten salt poured to a level above that of the side il. After the salt has solidified, the portion extending above the sides Il may be chipped off or sawed oiI flush. The cover IB is then sealed to the casing.

The mineral filling is in a good heat transfer relation to the electrical members after solidification. The recrystallized mineral possesses high thermal conductivity to dissipate the heat with a low thermal gradient between heated members and the casingY The electrical conductors 26 may be made available for connecting with the bushings 22 and 24 by attaching a removable protecting plug or block thereto prior to the casting of the salt. Upon cutting or removal of the surplus salt, the blocks or plugs may be removed and conductors 28 will be exposed.

In an experimental setup embodying this expedient of casting a salt about electrical conductors, it has been found that the copper is not attacked by the salt, but retains a very bright and unoxidized appearance. The elements are protected to a very high degree from damage by oxidation and other influences. The presence of moisture with the salt obviously would cause corrosion, and it is an object of the sealed casing construction to prevent this occurring.

The transformer construction as disclosed in the drawing is much more satisfactory for cooling electrical elements with Vrespect; to a lower thermal gradient than is air cooling. In cornparative tests of substantially identical' apparatus, it was found that better heat dissipation was effected by the use of a solid cast salt filling around the electrical element than by the use of air cooling.

In certain installations, the possibility of the alternate expansion and contraction of the electrical elements as they are energized and deenergized may result in some fissuring or cracking of the cast block of crystalline salt or mineral. In such cases, it would be satisfactory to place a small quantity of an insulating liquid,` such as a halogenated cyclic hydrocarbon, which is relatively non-inflammable and explosion-proof, within the shrink head cavity 42. In some instances, a viscous material, such as an asphaltic compound, may be placed in the shrink cavity. The halogenated liquid material or compound will enter the fissures or cracks and provide for good thermal conductivity between the separated faces of the salt.

It will be appreciated that thehalogenated hydrocarbon or asphaltic compound will not be suitable for operation at extremely high temperatures at which the mineral alone would be satisfactory. However, the minerai is so inexpensive as compared to the liquid halogenated dielectric that a considerable saving may be effected as compared to an all liquid filling by employing a small proportion of liquid dielectric with a maior proportion of salt.

In certain instances, the casing Il of the transformer may be provided with an expansion joint so that the salt may be cast within the casing when the casing has been mechanically expanded, and after the salt solidifies the casing upon release and contracting will maintain a continuous, close contact about the body of solid material. These considerations relate to the design of the casing to provide for various contingencies that may be met with under exceptional operating conditions.

Since certain changes may be made in the above description and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the vabove description shall be considered as illustrative and not in a limiting sense.

I claim as my invention:

1. Electrical apparatus comprising, in combination, an electrical member subject to heating when in operation, and a heat conducting dielectric body surrounding and embedding the electrical member to provide for spacing the member, and for dissipating the heat, the heat conducting dielectric body composed essentially of a substantially homogeneous crystalline mineral solid cast about the member while in a molten state, the mineral solid selected from the group consisting of sodium chloride, potassium chloride, calcium fluoride, sodium fluoride and potassium fluoride, and providing for a good transfer of heat from the embedded electrical member to the mineral solid and a rapid conduction of the heat to the exterior of the body in order to maintain predetermined temperatures in the electrical member, the cast mineral being so disposed about the electrical member that it constitutes the major electrical insulation therefor.-

2. Electrical apparatus comprising, in combination, an electrical member subject to heating when in operation, a sealed casing enclosing and protecting the electrical member and a solid heat conducting dielectric body filling the casing and embedding the electrical member to provide for electrically insulating the member, the solid heat conducting body composed essentially of a solid inorganic substantially homogeneous material having a thermal conductivity of 0.01 calorie or higher per second per degree centigrade per eter thickness per square centimeter, the inorganic material selected from the group consisting of sodium chloride, potassium chloride, sodium iuoride, potassium fluoride and calcium fluoride, cast about the electrical member while in the molten state and solidified to a crystallice mass and substantially conforming to the casing, the whole being associated to provide for D, good transfer of heat from the embedded elecicel member to the solid body, a good conduc- 'cn of the heat to the exterior of the body and transfer of the heat from the body to the seing in order to provide for a low thermal radient throughout the electrical apparatus.

3. In. electrical apparatus, in combination, an electrical member subject to heating when in 

